1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system with a function to locate a fault on an optical transmission line.
2. Description of the Related Art
The international telecommunication needs are rapidly growing in recent years, with the major driving forces being business globalization and Internet proliferation. In such circumstances, submarine optical transmission systems are expected to be as important as satellite communications systems, to meet the increasing demands for more cost-effective, bandwidth-rich telecommunication service.
In submarine optical transmission systems, in-line repeaters are placed at certain intervals along the fiber optic cable laid under the sea, so that the attenuation losses of signals will be compensated for by intermediary optical amplification. Such submarine systems are required to offer the highest level of reliability because a trouble in the undersea equipment would cost a lot of time and money to repair. For this reason, the system must have an integral troubleshooting mechanism that can pinpoint the fault in the event of such a failure.
To ensure that the optical transmission line is operating correctly, the monitoring system stimulates communication between end stations and repeaters. More specifically, an optical monitoring command signal is sent from an end station on the land to repeaters under the sea, so as to observe the operating status of each individual repeater. Upon receipt of such a command, the repeaters collect information about their own operating status and send the resultant response message back to the requesting end station.
Monitoring response signals are conveyed over the optical transmission line, being superimposed on main optical transmission signals. Conventional repeaters use an erbium-doped fiber amplifier (EDFA) to boost optical transmission signals with a pump beam generated by laser diodes, and they modulate the pump beam with their monitoring response signals to overlay the information on an optical transmission signal directed to the end station. Even when the incoming optical signal was lost due to a fiber failure or damage, EDFA-based repeaters could respond to monitoring command signals because they can send a response signal by modulating amplified spontaneous emission (ASE) waves produced from the EDF amplifier itself.
While EDFAs are commonly used in WDM systems, researchers have studied several other optical amplification techniques. Raman amplifier is one of the most recent and interesting developments in the field of optical communications, which is based on a nonlinear effect in optical fiber, known as the “Raman effect,” that the wavelength of light changes when a light beam is deflected by vibrating molecules. Signal amplification occurs if optical pump waves with the correct wavelength and power are launched into the optical fiber, turning the full transmission length into an amplifying medium.
Such Raman amplification techniques can be applied to optical repeaters to realize a longer-distance, higher-capacity fiber-optic transmission system. As in the EDFA-based systems, repeaters with Raman amplifiers send monitoring response signals to report their operating status to an end station by modulating the pump laser diode output with a response message signal. The monitoring response signals are delivered to the requesting end station as a modulated component of the main optical transmission signals.
Repeaters using Raman amplification, however, would have a serious problem when a fiber failure or damage occurs at a point relatively close to them. As noted before, Raman amplifiers use fiber optic cables as the amplifying medium, meaning that a near-point fiber failure would detach that amplifying medium from the repeater. Since there is neither incoming optical transmission signals nor ASE being produced, the repeater has completely lost the vehicle for its monitoring response signals. The end station cannot monitor the repeaters.